1. Field of the Invention
This invention relates to devices and methods for non-invasive optical physiological measurements, including the detection of a photoplethysmography (PPG) signal from a user, and more particularly to a pressure detection assembly of an optical measurement device which detects an amount of pressure applied by the user to the device, and a feedback unit which aids the user in determining an optimal PPG signal.
2. Description of the Related Art
Optical monitoring of physiological characteristics utilizes the detection of light transmitted through a location of a user being measured. Photoplethysmography (PPG) is an optical measurement technique used to detect blood volume changes in the microvascular bed of living tissue, typically by detecting light transmitted through the ear lobe or fingertip. As arterial pulsations enter the capillary bed, changes in the volume of the blood vessels or characteristics of the blood itself modify the optical properties of the capillary bed. The PPG signal is used to measure saturation of peripheral oxygen (SpO2), which is an estimation of the level of oxygen saturation in a fluid, such as blood. The PPG signal can also be used to measure blood pressure.
A device such as a pulse oximeter is an accepted standard in clinical practice, and provides for measuring enhanced optical pulsatile signals emitted by the changes in the volume of blood flowing through a user. The pulse oximeter generally has a pair of small light emitting diodes (LEDs) facing a photodiode, with a translucent part of the user's body, usually a fingertip or an earlobe, positioned there between. The light from the LEDs passes through the tissue and is detected by the photodiode. One LED is red, with wavelength of approximately 660 nanometers (nm), and the other is infrared, with a wavelength of approximately 905, 910 or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form. Therefore, the ratio of oxyhemoglobin to deoxyhemoglobin can be calculated from the ratio of the absorption of the red and infrared light, i.e. the ratio of red light to infrared light absorption of pulsating components at the measuring site.
The basic form of PPG technology requires only a few optoelectronic components: a light source to illuminate the tissue (e.g. skin) and a photodetector to measure the small variations in light intensity associated with changes in perfusion in a catchment volume. FIG. 1 illustrates a graphical representation of a PPG signal 100, which can generally be divided into two components: an AC component 102 due to the absorption of light in pulsatile arterial blood volume 106; and a DC component 104 caused by the absorption produced by non-pulsatile arterial blood—i.e. venous blood and capillary blood 108, and tissue absorption 110.
In FIG. 1, this AC component 102 is superimposed onto a large quasi-DC component 104 that relates to the tissues and to the average blood volume. This DC component 104 varies slowly due to respiration, vasomotor activity and vasoconstrictor waves. With suitable electronic filtering and amplification, both the AC component 102 and DC component 104 can be extracted for subsequent pulse wave analysis.
Two important characteristics of the PPG AC pulse waveform 102 have been described and are illustrated in FIG. 2, where the appearance of the pulse waveform was defined as two phases: a first anacrotic phase 112 being the rising edge of the pulse, and a second catacrotic phase 114 being the falling edge of the pulse. The first phase 112 is primarily concerned with systole, while the second phase 114 represents diastole and wave reflections 116 from the periphery. A dicrotic notch 118 is usually seen in the second catacrotic phase 114 of subjects with healthy compliant arteries.
The majority of PPG devices currently available rely on simple thresholding, or peak detection algorithms, to find the principal peaks in a detected signal. However, these methods are unreliable when the detected signal is less than ideal. Particular problems may be encountered when the baseline of the AC signal component becomes noisy or complex, as can occur even with mild movement artifacts.